1. Field of the Invention
The invention generally relates to an information signal reproducing apparatus for a disc record and more particularly, this invention relates to an information signal reproducing apparatus suitable to reproduce digital data recorded on the disc.
2. Description of the Prior Art
A disc is known on which an audio signal and other information signal which are digitized are recorded. By way of example, for the audio signal, such an optical disc is known on which the audio signal is pulse code modulated (PCM) and then recorded. By the way, to record a pulse code modulation (hereinafter simply referred to PCM) signal on the disc, a method of recording at a constant angular velocity and a method of recording at a constant linear velocity are proposed. In view of the increase of the recording density, the recording at the constant linear velocity is preferable. The disc on which the PCM signal is recorded at the constant linear velocity has to be reproduced at the constant linear velocity.
Such a method for controlling the disc rotation at the constant linear velocity upon playback is known that a position of a pick-up is detected by a potentiometer and since the necessary rotational number becomes the inverse number of such position, the detected output is supplied to a divider so as to obtain a control information. But, this method needs such an arrangement consisting of the position detector or potentiometer and the divider, so this leads to expensive and complex arrangement.
Therefore, in order to overcome the two defects as mentioned above, it is proposed to use the reproduced signal from the disc, without using the detector for detecting the position of the pick-up, so as to control the disc rotation at the constant linear velocity.
For instance, in a digital audio disc of an optical signal detection system, the optical disc is usually manufactured by the following processes stated below; a mastering process to form an original disc on which pits (recesses) corresponding to "1" or "0" of a recorded signal by the use of a laser beam optically modulated by the recorded signal; a process for duplicating the digital audio disc from this original disc by the method same as that of the normal analog disc. But, in this case, depending upon the condition of the mastering process and the like, the sizes of the pits are displaced or shifted uniformly by a predetermined amount so that a phenomenon where even when the ON/OFF ratio of the recorded signal is 50%, the ON/OFF ratio of the reproduced signal does not reach 50% (termed as asymmetry) will occur. In other words, when in a waveform converting circuit of a reproducing system, the reproduced signal is converted to the pulse signal, the pulse width thereof differs from that of the recorded signal. As a result, this causes such a problem that the processings of demodulating the reproduced data and so on will not be performed correctly. In the known reproducing apparatus, when the signal read out from the disc is supplied to a comparator used as a waveform converting circuit so as to be waveform-converted, a reference level for comparison (threshold level) is adjusted manually for the purpose of overcoming the aforesaid problems. Thus the adjusting operation is quite troublesome.
Moreover, when the audio signal is digitized, for example, converted to a PCM signal to be recorded, the audio signal is recorded at the base band system which is not a carrier modulation system such as amplitude and frequency modulations. In this case, a modulation method such as a run length limited code is used normally. This run length limited code modulation method is, with respect to data "0" or "1", a minimum transition interval T.sub.min between two data is extended to enhance the recording efficiency and a maximum transition interval T.sub.max therebetween is shortened to facilitate a self clock on a playback side.
Then, the deviation of the maximum or minimum transition interval from a reference value where the linear velocity is taken as a reference is detected and used as an information to correct or compensate for the velocity servo and the asymmetry.
In other words, the above correcting or compensating means includes a peak value holding circuit for deriving the maximum transition interval T.sub.max appearing successively twice from the signals reproduced from the disc and peak-holding the interval, another peak value holding circuit for inverting the above maximum transition interval T.sub.max and peak-holding therein the inverted signal, and a circuit for deriving a difference between the outputs of both the peak value holding circuits, in which the differential output mentioned above is employed as the signal to compensate for the asymmetry. Also, based upon a clock component contained in the signal reproduced from the disc, a PLL (phase locked loop) circuit is driven. The output of this PLL circuit and a clock from a reference clock oscillator are compared with one other to permit the motor driving to be controlled.
The method as described above has the control signal system formed in analog fashion so that it has a disadvantage that the accurate correction or compensation of the asymmetry or the accurate control of the motor velocity and phase is impossible.